The present invention relates to a process and apparatus for catalytic cracking of hydrocarbon feeds.
The petroleum industry routinely employs cracking processes in which hydrocarbon molecules with a high molecular weight and boiling point are split into smaller molecules with a lower boiling point.
In recent catalytic cracking processes such as that described in European patent EP-A-0 291 253, the cracking reaction takes place in an elongate vessel with a substantially circular cross section, the catalyst being admitted into the lower portion of the vessel along with the atomised hydrocarbon feed. Bringing the feed into contact with the hot catalyst vaporises the hydrocarbons, entraining the catalyst towards the upper portion of the reaction zone, with the introduction of an entrainment fluid assisting the upward movement. The products formed during the reaction have a very broad range of boiling points. In general, the products formed are distinguished by their boiling point and chemical nature:
The boiling points delimiting the cuts are given by way of indication and correspond to generally accepted standard values. Those cut points can vary depending on the refiner""s needs, and in some cases can also produce intermediate cuts from the products formed.
The yields that are generally obtained naturally depend on the quality of the treated feeds. Typically, by way of indication, the yields observed (as a % by weight of the feed) for units are:
In general, the coke formed is burned in one or more vessels termed regenerators towards which the catalyst circulates from the reactor outlet. The heat produced by combustion of the coke re-heats the catalyst, which is then re-introduced into the reactor inlet and brought into contact with the feed. The catalytic cracking process is an adiabatic process. The heat recovered by the catalyst during its passage into the regeneration zone is equal to the heat lost by the catalyst during its passage through the reaction zone. This constrains the operator to employ operating conditions that are not independent of each other. The operating conditions that most affect the yields and selectivities for a given reactor are essentially the catalyst flow rate, which is generally related to the feed flow rate by the term C/O (C for catalyst, and O for oil). The normal range for operating catalytic cracking units is generally:
C/O=4-8 (C/O=weight ratio of catalyst flow rate to feed flow rate);
T (at reactor head)=500-550xc2x0 C.
Conversion is known to increase with temperature and C/O. However, using conventional techniques, that increase can, inter alia, be accompanied by a significant increase in the coke and dry gas yields. Increasing the coke yield by means of the regenerator-reactor thermal balance and dimensions of the unit frequently limits the operator to a restricted range of operating conditions and, for a given feed type, to a fairly fixed yield structure.
The sale price of different products can fluctuate with time, which may tempt the refiner to decide to maximise certain products to the detriment of some others. Further, the change in specifications imposed on the products in different states means that certain FCC products may no longer have an outlet (for example, LCO is highly aromatic and has a very poor cetane index, so its use in certain fuels in the gas oil pool poses a problem; the sulphur content of heavy gasoline (160xc2x0 C.-220xc2x0 C.) renders its use in gasoline pools difficult in some cases). It may thus be advantageous to minimise certain cuts as well.
Maximising propylene, a product with a high added value (molecule included in the LPG cut) is known to involve making the reaction conditions more severe (higher temperature, higher C/O). At the same time, making the conditions more severe means that the yields of other cuts (LCO and gasoline) decreases.
Mild conditions tend to maximise the LCO, which may be advantageous in states in which middle distillates are in great demand in the fuel market, but LPG (including propylene) and gasoline will probably not be maximised.
Thus, the operation of the reaction zone for a conventional unit is not always compatible with achieving the two aims such as the following non-limiting examples:
maximising propylene and LCO;
minimising heavy gasoline, maximising light gasoline.
There is thus a need for solutions that can enable the reaction zone to operate both under severe and mild conditions, for example using two reaction zones operating under different operating conditions.
The reaction zones generally used in the majority of catalytic cracking units in current use readily allow operations to be carried out under mild cracking conditions (C/O of 4 to 8 and reactor outlet temperatures of 500xc2x0 C. to 550xc2x0 C.). The residence time for the hydrocarbons in that reaction zone, the minimum constitution of which is a tube with a substantially circular cross section and elongate form in which the fluids flow in an overall bottom to top direction, usually termed a riser, and a system for separating the cracked vapours and the catalyst, is generally more than 2 seconds (s), of the order of about 2 s to about 10 s. The residence time for the hydrocarbons in contact with the catalyst is usually more than 1 s.
Juxtaposing two conventional reactors to obtain two types of operating conditions in the same catalytic cracking unit such as that described by Niccum, P. K., Miller, R. B., Claude A. and M. A. Silvermann in xe2x80x9cMaxofin: a novel FCC process for maximizing light olefins using a new generation ZSM5 additivexe2x80x9d (1998, NPRA annual meeting, San Francisco, Calif., USA, Mar. 16th, 1998), renders necessary the use of additives in the second riser where the reaction is carried out under more severe conditions to obtain a more favourable selectivity. Further, the more severe conditions in the second reactor cause a very large increase in the coke yield (more than 2% with respect to the feed). The arrangement of that type of system is thus not optimal.
The prior art is also illustrated by U.S. Pat. Nos. 4,424,116 and 4,606,810, which describe a concatenation of two riser reactors in series. U.S. Pat. No. 5,039,395 also illustrates the prior art.
In order to minimise a cut, with a unit possessing one or more conventional riser type vessels, it is also possible to recycle the products the production of which is to be minimised to the riser, in the case of heavy feeds, this has a huge advantage for the thermal balance of the units: vaporising the recycle consumes more heat and thus produces more heat in the regeneration zone and thus more coke in the reaction zone; further, since it is cleverly located downstream with respect to fresh feed injection, injection of the recycle encourages fresh feed vaporisation which then enables even heavier feeds to be treated (with higher median boiling points and end points). Such an apparatus has, for example, been described in French patent FR A-2 621 322 for cracking heavy cuts.
In that type of implementation, the recycled products are not exposed to very severe conditions and react only slightly. The aim of the recycles has more to do with the thermal balance and vaporising the feed than degrading the recycle into higher added value products.
It is also possible to use a recycle upstream of the feed, to expose the recycle to conditions that are more severe than those for the feed. Under those conditions, the products formed under the most severe conditions have the time to degrade above the feed injection where the residence time in contact with the catalyst is necessarily quite long (more than 1-2 s).
In order to operate under more severe operating conditions, it is preferable to use shorter hydrocarbon residence times in the reactor. By increasing the temperature, thermal degradation of the products becomes increasingly important. To limit their impact, the residence time in the hydrocarbons under such conditions must be limited. Further, the residence time is short and still further, the mechanisms controlling contact between the hydrocarbons and catalyst must be properly controlled along with the hydrodynamics of the reactor. The riser reactor combined with a suitable mixing system such as that described in International patent application WO/FR98/122279, can optimise the selectivities for higher value products (LPG, gasolines) by minimising the zero value products (minimal increase in coking compared with a conventional reactor, but under very different temperature and C/O conditions, a reduction of about 30% in dry gases compared with conventional technology) and maximising conversion, thanks to the production of very severe conditions.
It can thus be envisaged, in order to increase the flexibility of the FCC operation, to provide a concatenation of a dropper reactor with a riser reactor. However, according to EP-B-0 573 316 describing that apparatus, all the products exposed to the riser reactor must then be passed into the riser reactor. The residence time for the products formed in the dropper reactor is thus prolonged by the that for passage through the riser reactor. Further, it is not suggested that these two reactors could be operated under significantly different operating conditions.
The essential advantage of this type of apparatus is to be able to bring the catalyst and feed into contact in an optimal manner due to the initial use of a dropper reactor.
When carried out correctly and when the contact time between the catalyst and the hydrocarbons is limited, bringing hydrocarbons into contact with the catalyst in a dropper reactor can minimise the quantity of coke formed. This results in a much lower quantity of coke on the catalyst than in the equivalent riser reactor. Combined with suitable operating conditions (higher circulation rate for the catalyst compared with the same quantity of feed), meaning that the quantity of coke on the catalyst can be very significantly reduced, which is of particular advantage for heavy feeds which are known for their coking ability. Further, coke deposited on the catalyst tends to significantly deactivate the catalyst; the more coke, the more significant the deactivation. Typically, in conventional riser reactors, the quantity of coke present on the catalyst is between 0.7% and 1.5% by weight, depending on the feed treated, the catalyst, the operating conditions and the dimensions of the unit. Under such conditions, the residual activity of the catalyst is known to be low. It is thus foolish to wish to re-introduce the catalyst into a new reaction vessel. In contrast, in the case of a dropper reactor, it is possible to limit the quantity of coking on the catalyst to values of about 0.2-0.5% by weight depending on the operating conditions, solution its residual activity remains high. Under such conditions, the catalyst from the dropper reactor can advantageously be introduced into a reaction chamber such as a riser, optionally mixed with a flow of regenerated catalyst (i.e., directly issuing from the regeneration chamber). It can thus be seen that a concatenation of reaction zones that are initially in dropper mode, then in riser mode can readily be envisaged where the catalyst from the dropper reaction zone is re-introduced in its entirety into the inlet to the riser reactor.
The aim of the present invention is to remedy the omissions of the prior art by proposing a concatenation of distinct reaction zones that can operated under very different temperature and C/O conditions. More precisely, the invention concerns a catalytic cracking process composed of a reaction zone with at least two reactors, at least one of said reactors having an overall downward flow of fluids and catalyst (dropper reactor) and at least one of said reactors having an overall upward flow of fluid and catalyst (riser reactor), said reactors being characterized in that in each reactor, the hydrocarbons introduced into the reactor are brought into contact with hot catalyst to vaporise said hydrocarbons if they are introduced in the liquid form, said vaporised hydrocarbons then reacting in the presence of the catalyst, the reacted hydrocarbons then being separated from the catalyst using separation means (inertial separators and/or cyclones) and leaving the reaction zone to undergo routine downstream treatments (fractionation, . . . ). The reactors are also characterized in that the dropper reactor or reactors is/are followed by at least one riser reactor, all of the catalyst from the dropper reactor(s) passing into at least one downstream riser reactor.
More particularly, the invention provides an entrained bed or fluidised bed process for catalytic cracking of a hydrocarbon feed in two reaction zones, one zone in catalyst dropper mode, the other in catalyst riser mode, the process being characterized in that a feed and catalyst from at least one regeneration zone are introduced into the upper portion of the dropper zone, the feed and catalyst are circulated in said zone in a catalyst to feed, C/O, weight ratio of 5 to 20, the cracked gases and coked catalyst from the dropper zone are separated in a first separation zone, the cracked gases are recovered, the coked catalyst is introduced into the lower portion of the riser zone, a feed is introduced into the lower portion of said riser zone, the coked catalyst and said feed are circulated in a C/O weight ratio of 4 to 8, the used catalyst is separated from the effluent produced in a second separation zone, the catalyst is stripped using a stripping gas in a stripping zone, the effluent and stripping gases are recovered and the used catalyst is recycled to the regeneration zone where it is at least partially regenerated using a regeneration gas.
The residence time for the feed in the dropper and riser are respectively generally 50 to 650 ms in the dropper and 600 to 3000 ms in the riser, preferably 100 to 500 ms in the dropper and 1000 to 2500 ms in the riser. The residence time is defined as the ratio of the volume of each of the reaction vessels (riser or dropper) with respect to the volume flow rate of the gaseous effluents in each chamber under the outlet conditions.
According to one characteristic of the process, the used catalyst is regenerated in two superimposed regeneration zones, the used catalyst to be regenerated being introduced into a first lower regeneration zone, the at least partially regenerated catalyst being sent to the second, upper regeneration zone and the regenerated catalyst from the upper regeneration zone being introduced into the dropper reactor.
The catalyst to oil (C/O) ratio is advantageously in the range 7 to 15 for the dropper reactor and in the range 5 to 7 for the riser reactor.
The temperature of the catalyst at the dropper outlet is generally higher than that at the riser outlet. It can be 500xc2x0 C. to 700xc2x0 C., advantageously 550xc2x0 C. to 600xc2x0 C., while that of the catalyst at the riser outlet can be in the range 500xc2x0 C. to 550xc2x0 C., advantageously 515xc2x0 C. to 530xc2x0 C. These temperatures are strictly dependent on the respective values of C/O, the C/O ratio of the dropper being higher than that of the riser.
According to a characteristic of the process, the feed supplying each of the reactors can either be a fresh feed, or a recycle of a portion of the products from downstream fractionation, or a mixture of the two.
Preferably, a fresh feed can be introduced into the riser reactor and at least a portion of the recycle can be introduced into the dropper reactor.
It may be advantageous to introduce the feed into the riser reactor above the point of introduction of the coked catalyst and the point of introduction of the regenerated catalyst.
The feed can be injected into each of the two reactors as a co-current or counter-current.
According to one characteristic of the process, the feed flow rate, for example the recycle, into the dropper reactor can represent less than 50% by weight of the feed flow rate to be converted circulating in the riser reactor.
The invention also concerns an apparatus for carrying out the process. It generally comprises:
a first substantially vertical dropper reactor with an upper inlet and a lower outlet;
a first means for supplying regenerated catalyst connected to at least one regenerator for used catalyst and connected to said upper inlet;
a first means for supplying atomised feed disposed below the first catalyst supply means;
a first vessel for separating catalyst from a gas phase connected to the lower outlet from the first dropper reactor and having an outlet for a gas phase and an outlet for coked catalyst;
a second substantially vertical riser reactor having a lower inlet and an upper outlet;
a second means for supplying catalyst connected to the outlet for coked catalyst from the first separation vessel and to the lower inlet to the second reactor;
a second means for supplying feed located above the lower inlet into the second reactor;
a second vessel for separating used catalyst from a second gas phase connected to said upper outlet from said second reactor, said second chamber comprising a catalyst stripping chamber and having an upper outlet for a gas phase and a lower outlet for used catalyst, said lower outlet being connected to the regenerator.